Methods and apparatus are known in the art which are used to cope with the problem of limited resources on the one hand and the increased number of mobile subscribers on the other. To overcome this problem different solutions are imaginable. These can be roughly classified in hardware solutions, improved channel allocation schemes and the change of handover boundary parameters.
Of course digital hardware i.e. resources and channels or cells, respectively can be installed at locations where the network is expected to become overloaded. Yet, additional channels need to allocate new bandwidth which is not available for most of the operators. Moreover, additional cells require new locations, which are either not possible to acquire or are very expensive to install. Both methods do not exploit the unused capacity in lightly loaded cells. Another hardware solution could be to employ adaptive antennas for a dynamic distribution of the traffic. Furthermore, the problem of unbalanced traffic could be solved if modern relay technologies were integrated in the cellular infrastructure. However, the latter method is even more cost effective than installing additional base stations for additional cells.
Moreover, dynamic channel allocations schemes are proposed which are capable of adapting the network to changing load situations. Contrary to the fixed channel allocation, which is mostly used in today's mobile communication systems, the new methods are based on the provisioning of a pool of available channels which are dynamically exposed and assigned to the cells. Thus, in highly loaded cells more channels can be assigned than in lightly loaded cells. However, additional channels are quite expensive and the method implies a heavy control function.
Since the traffic load tends to vary during the day, e.g. according to rush-hours, many cells in the cellular network are loaded differently over time. However, some cells will temporarily enter a congested state while other cells have only a low load. On the one hand, the highly loaded cells operate at a high blocking probability, which corresponds to the local capacity limit. On the other hand, the low loaded neighboring cells have some unused capacity, which conventionally can not be exploited to absorb traffic of the highly loaded cells. Therefore, there is some room for improving the system capacity by equalizing the cell load all over the network by exploiting the cell capacity in lightly loaded cells. The problem can be observed in a microcellular environment as well. Especially, as the cell size gets smaller the unequal distribution of the subscribers becomes very likely.
One possibility to equalize the cell load addresses the dynamic adaptation of handovers. The dynamic adjustment of the handover boundaries is a very simple method for increasing the overall capacity of the network without the necessity of adding new resources.
There are already state of the art proposals in place to equalize the cell load. Yet, these methods usually use only the current load situation for deciding whether or not a call is to be directed to a neighbor cell. However, if the load situation is completely different in the next time interval (when the load situation is updated), the cell borders are immediately shifted back. This effect leads to rapid variations of the cell boundary. Thus, it becomes probable that a lot of unnecessary or ping-pong handovers occur, increasing the signaling load on the base station controllers. As there is always a certain probability for handover failures, the overall network performance will be degraded due to the increased number of handovers. Since the increased number of handovers increases the signaling load on the network and degrades the speech quality, it is obvious that the method deciding on handovers plays a key role in modern communication systems.
Therefore, it is in particular an object of the present invention to provide a method for improved load balancing between cells in combination with an improved handover strategy.